Air heater fuel control system



Jan. 16, 1962 P. J. AMUNDSON AIR HEATER FUEL CONTROL SYSTEM 54QMAGNETOFiled Sept. 29, 1958 FUEL FILTER INVENTOR. PAUL J. AMUNDSON ATTORNEY3,617,112 ATP. HEATER FUEL CONTROL SYSTEM Paul J. Amundsen, Moline,[1]., assignor to American Air Filter Company, inc, Louisville, Ky., acorporation of Delaware Filed Sept. 29, 1958, Ser. No. 763,967 7 Claims.(Cl. 236) This invention relates to portable air heaters employingliquid fuel and relates particularly to a fuel control system for such aheater.

Hubbard US. Patent 2,758,591, issued August 14, 195 6, exemplifies aportable space heater used extensively for supplying heated air for amultitude of purposes. The present invention is directed to a fuelcontrol system for an air heater of this general type.

Such heaters conventionally include: a burner or combustor to whichliquid fuel is supplied and ignited; a heat exchanger to whichventilating air is delivered for tempering; an outlet for the temperedventilating air; a ventilating air temperature sensing element forsensing the tempered air; and means for controlling the flow of fuel tothe burner in accordance with the differential between a selected airtemperature and sensed air temperature.

Under fortuitous firing conditions, the control system may hunt as aresult of the time lag between the sensing by the controls of anincorrect outlet temperature and a change or correction of thetemperature condition of the heat exchanger. Such hunting may occur, forexample, where the fuel is being fired at a rate producing an outlettemperature exceeding the selected temperature, and consequently, thesensing element signals or demands a decreased fuel discharge rate intothe combustor. Before this decreased fuel discharge rate is manifestedat the sensing element, however: (1) the flow controlling means mustrespond; (2) the mass of the heat exchanger must decrease intemperature; and, (3) the decreased ventilating air temperatureresulting from the foregoing must be sensed by the sensing element.While these successive steps are occurring, it frequently happens thatthe flow control means overshoots a position which would have providedthe correct fuel discharge rate. In other words, the firing rate in thecombustor becomes lower than the rate which would have satisfied thesensing element at the outlet of the heater. As a result, a reversesituation prevails (outlet temperature sensed is below selectedtemperature), and the same steps occur in an effort to increase theoutlet temperature to the selected value. This hunting situation maycause undesirable swings in outlet temperature.

The present invention has, as one of its objects, the provision of acontrol system which substantially minimizes hunting of the controlsystem when the system responds to an outlet temperature exceeding theselected temperature.

A more specific object is the provision of valve means operative torapidly reduce the rate at which fuel is discharged into the combustorwhenever the control means calls for a decreased fuel rate in responseto a sensed temperature exceeding the selected temperature.

It is a further object of this invention to provide a control systemhaving such valve means with means for rendering the valve meansinoperative with the system as a whole is operating under conditionswhere the main fuel control valve is in a minimum fuel flow position andfuel flow to the combustor is being controlled in an onoff method ofoperation as opposed to a modulating control of fuel flow.

Another object is the provision of a fuel control system in which thevalve means for rapidly reducing the fuel discharge rate into thecombustor is arranged in parallel 3,017,112 Patented Jan. 16, 1962 withthe main fuel control valve and which is operative to restore fuel flowcontrol to the main fuel flow control valve when the air temperaturesensitive means are apparently satisfied.

These and other objects are attained in accordance with the invention byproviding a system which includes a main fuel flow control valveoperative in a modulating type of operation in response to thedifferential between a sensed and selected temperature and a valve inparallel with the main valve for permitting the flow of fuel around themain valve and thereby rendering the main valve control ineffective whenthe parallel valve is open. The parallel valve is arranged to beoperated to an open position whenever the air temperature sensitivemeans controlling the main valve demands a lower fuel rate. As a result,when such lower fuel rate is demanded, the fuel is bypassed through theopen parallel valve andthe fuel discharged into the combustor is rapidlyreduced so that the decreased outlet temperature is obtained morequickly than if the main valve alone controlled the rate of fueldischarge into the combustor. The system of control also serves tocompensate for the overshooting tendency of the main valve due to heattransfer lag factors.

Additionally, in accordance with this invention, this system is readilyadapted for utilization in connection with a fuel control system of thetype disclosed and claimed in Robson US. patent application Serial No.760,359, filed September 11, 1958, of the same assignee and whereinprovision is made for modulating control of fuel flow in a relativelyhigher temperature range, and, on-off control in a lower range. In theadaptation of the present invention to such a system means are providedto maintain the parallel valve in a closed position in response to acondition of the control system calling for the oil-off control.

The invention will be described in connection with the accompanyingdrawing illustrating a preferred embodiment of the invention by way ofexample, and wherein:

FIGURE 1 is a diagrammatic view illustrating the heater fuel flow systemand certain elements associated therewith for controlling fuel flow andheater operation;

FIGURE 2 is a diagram of an electrical circuit adapted to provide thecontrol for that part of heater operation to which this invention isdirected.

The combustion and air heating system is of the general type illustratedin Hubbard US. Patent 2,758,591, issued August 14, 1956, and as showndiagrammatically in FIG- URE 1 comprises: forced air blower means 2 toprovide ventilating air and combustion air; a combustion chamber orburner 4 adapted to receive combustion air from the blower 2 and liquidfuel from jet nozzle 6, the burner also containing igniting sparkelectrodes 8 and communieating with an exhaust stack 10 for dischargingthe exhaust gases; an outer jacket 12 defining a heat exchangingpassageway between the combustion chamber and the outer jacket so thatthe ventilating air may be heated in its passage therethrough, the outerjacket terminating in an air outlet 14 through which ventilating air isdischarged into suitable conveying ducts for delivery to the servicedspace; and a temperature control sensing element 16, and a temperatureoverheat sensing element 18, both of which are associated withWheatstone bridge circuits to be hereinafter described in some detail.

The fuel flow system will now be described. Fuel is drawn from tank 20through conduit 22 and the fuel filter 24 by pump 26. A regulating valve28 on the discharge side of pump 26 is a conventional balancedregulating type valve within housing 30 and is adapted to open and tosupply fuel at a predetermined pressure to nozzle supply conduit 32.When the regulating valve 28 opens, fuel flows at that predeterminedpressure (e.g., psi.) through conduit 32 to nozzle 6. Secondary by-passcon- 3 duit 34 returns fuel, in excess of that required to maintain asteady 150 psi. in nozzle conduit 32, back to tank 20. Primary bypassconduit 36 contains a two-position (open-closed) valve 38 controlled bysolenoid 39.

Burner nozzle 6 is of the by-pass type which permits an operationwherein the rate of fuel discharged into the burner through nozzle 6 iscontrolled by throttling on the downstream side of the nozzle. Such aburner nozzle is illustrated and described in detail in Hubbard US.Patent 2,758,591 and includes a supply chamber into which fuel fromconduit 32 fiows, a combustion jet orifice through which some of thefuel escapes as a spray or jet into the burner, and a by-pass chamberwhich receives the rest of the fuel. With fuel supplied by conduit 32 ata constant pressure to the nozzle, part of the fuel will be dischargedthrough the jet orifice into the burner chamber and part of it willby-pass to the by-pass chamber. The by-pass chamber connects to thenozzle bypass conduit 40 which contains a check valve 42 and athrottling valve 44. Thus, in operation, the more the nozzle by-passconduit is throttled by valve 44, the more fuel is discharged throughthe nozzle jet orifice; and conversely, the less the nozzle by-passconduit is throttled, the less fuel issues through the nozzle jetorifice. The downstream side of the throttling valve 44 is connected topass excess fuel back to the tank by way of return conduit 46. Thethrottling valve is modulated between end travel limits by reversiblemotor 48 which, in turn, is controlled in a manner to be describedhereinafter.

Valve 50 is a two position (open-closed) valve connected in parallelwith throttling valve 44 between the downstream side of check valve 42and return conduit 46 and controlled by solenoid 51. Valve 50 is biasedby conventional means to a closed position when solenoid 51 is notenergized, and is actuated to an open position by energization of thesolenoid. It will be apparent that while the valve 50 is closed, controlof the fuel discharge rate into the burner will be effected wholly bythe throttling valve. However, when valve 50 is actuated to an openposition, most of the fuel will by-pass valve 44 and fiow to returnconduit 46 by way of the relatively unrestricted path through open valveSll-the exact division of fiow between the two valves depending upon thedegree to which valve 44 is open at the time valve 50 is actuated to anopen position. However, it is to be understood that valve 50 is sized toprovide a substantial reduction in fuel back pressure to the nozzle 6when the valve is opened so that a correspondingly substantial reductionof fuel discharged into the burner occurs upon opening of the valve.

Fuel pump 26 is driven by electric motor 52 which also drives theignition magneto 54 electrically connected to supply power for theigniting spark between electrodes 8 at the nozzle orifice. This insuresan igniting are at the nozzle at all times when the fuel pump isoperating.

As will be apparent hereinafter from the description of the circuitutilized to control the fuel flow system, the solenoid operated valve 38is maintained in a closed position by energization of solenoid 39 whilethe throttling valve 44 is being modulated between its end travelpositions. Valve 38 is biased to an open position when the solenoid 39is de-energized. Thus, while the valve 38 is closed and the throttlingvalve 44 is between its end travel limits, the rate of fuel dischargefrom nozzle 6 into the burner is entirely under the control of thethrottling valve 44 and parallel valve 50.

Referring now to FIGURE 2, a suitable power source 58 is connectedthrough main switch 60 to provide power through line 62 to one side ofnormally open, manually closable switch 64. Closure of switch 64energizes line 66, relay winding 68, and motor 52 which drives fuel pump26 and ignition magneto 54. Energization of relay winding 68 operatesboth relay actuated switches 70 and 72 to the position illustrated inFIGURE 2 and thus through switch 70 line 74 serves to energize overheatbridge circuit 76 and temperature control bridge circuit 78.Energization of overheat bridge 76 causes current to flow throughpolarized winding 79 in a direction causing closure of overheat switch31 and thereby completes the alternate circuit for energizing relayWinding 68. Thus, when switch 64 is released by the heater operator,relay winding 68 remains energized through the overheat cutoff switch 80and relay actuated switch 70, and motor 52 remains energized throughline 66.

Switch 82 is controlled by current flow from temperature control bridge78 through polarized relay 84. Switch 82 is biased towards a neutralposition and is out of contact with its contact 86 and heating contact88 when current flow through polarized relay 84 in either direction isbelow a predetermined value. Current flow in one direction above thispredetermined value will cause switch 82 to close to cooling contact 86and thereby energize relay winding 90. It is noted that contact 86 andrelay winding 96 will be characterized as cooling contact and coolingrelay respectively since the effect of their operation is to reduce therate of heating. The use of the word cooling in this connection is notmeant to imply that any means for refrigeration or the like is broughtinto operation. Current flow in the opposite direction above thepredetermined value will cause switch 82 to close to heating contact 88and thereby energize relay winding 92.

Energization of relay winding 91], which may be characterized as thecooling winding, actuates switch 94 to its 94a position and closesswitch 95 as shown in FIG- URE 2 and indicates the bridge circuit '78 isdemanding a lower ventilating air discharge temperature. Energization ofrelay winding 92, which may be characterized as the heating winding,actuates switch 96 to its 96a position or opposite to the position shownin FIGURE 2 and indicates the bridge circuit 78 is demanding a higherventilating air discharge temperature. It will be apparent that bothwindings 90 and 92 cannot be energized at the same time and that whilebridge 78 is in substantial balance, thereby indicating the ventilatingair discharge temperature is relatively close to the desiredtemperature, neither winding 90 nor 92 will be energized.

Relay windings 98 and 101) may be characterized as holding relaywindings normally energized only during the time that on-off fuel flowcontrol is being employed. Energization of relay winding 98 actuatesswitches 162, 104 and 106 to their a contact positions. Energization ofrelay winding actuates switches 108 and 110* to their closed positionsand switch 111 to its open position as illustrated in FIGURE 2.

Switches 112 and 114 in the circuit to motor 48 are limit switches whichare actuated by travel of motor 48 to its end travel limits. Switch 112is actuated from its 1121? position to its 112a position when motor 48is driven to its end travel position corresponding to a maximum open(minimum stable flame-supporting fuel rate) position of throttle valve44. Switch 114 opens when motor 48 is driven to its opposite end travelposition corresponding to a maximum closed (maximum fuel rate to burner)position of throttle valve 44.

One side of switch 114 is connected by line 116 to contact 96a of switch96. Since the actuated element of switch 96 'is connected to power line74, when switch 96 is operated to its 96a contact by energization ofheating relay winding 92, the motor 48 will be energized and operated ina direction to closes throttle valve 44 and consequently throttle nozzleby-pass conduit 4%.

When switch 112 is inits 1126 contact position, and cooling relaywinding 91) is energized and holding relay winding '98 is de-energized,the motor 48 is connected by line 118, switch 104 in its 1041) position,line 129, switch 94 in its 94a position, line 122, and switch 1112 inits 1112b position to power line '74 and the motor 48 will be energizedand driven in a direction to open throttle valve 44 and consequentlyincrease the quantity of fuel by-passed through nozzle by-pass conduit40.

Contact 94b is connected by line 124 to contact 102a, contact 104a isconnected by line 126 to contact 106a and operable arm of switch 186 isconnected by line 128 to solenoid 39 which controls primary by-passvalve 38.

Contact 112a of limit switch 112 is connected by line 130 to one side ofswitch 72 which has its other side connected to holding relay winding 1%by line 132. Line 132 is also connected to one side of switch 108 whichhas its other side connected by line 134 to terminal 96b of switch 96and to one side of switch 110. The other side of switch 118 is connectedby line 136 to holding relay winding 98.

Switch 95 has one side connected to power line 74 and its other sideconnected by line 137 to switch 111. Switch 111 is connected to solenoid51 by line 139. Thus, when switch 95 is closed by energization ofcooling winding 90, and switch 111 is in a closed position due tonon-energization of holding relay 100, the circuit to solenoid 51 iscomplete and it is energized. If either switch 95 or 111 is open,solenoid 51 is de-energized.

Bridge circuit 78 includes the air discharge temperature sensingresistor 16 in one leg of the bridge and a variable temperatureselection resistor 138 in another leg of the bridge. Overheat bridge 76includes the overheat air discharge temperature sensing resistor 18 inone leg of the bridge and an overheat temperature selection resistor 140in another leg of the bridge. Both bridges 76 and 78 operate in aconventional manner wherein the direction and magnitude of current flowthrough the polarized relay windings 79 and 84 is dependent upon theresistance differential between the sensing and the selection resistors.

Operation The operation of the heater and control of fuel flow will nowbe described. After ventilation and combustion air flow is initiated,closure of switch 61) delivers power to one side of switch 64 which isthen momentarily closed to energize relay windings 68 and the motor 52.Energization of relay winding 68 actuates switches 70 and '72 to theirpositions shown in FIGURE 2 so that relay winding 68 will remainenergized through overheat safety switch 80 after switch 64 is released.With switches 79 and 72 closed, power is delivered by line 74 to both ofthe bridges, to the temperature control switch 82 and to various otherswitches as shown in FIGURE 2.

For purposes of explanation, it is assumed that motor 48 and valve 44are in an intermediate position, that limit switch 114 is closed, andlimit switch 112 is in its contact 11211 position so that the motor 48may be energized for movement in either direction. While a slow firestart of the heater may be accomplished through utilization ofadditional circuitry described and claimed in Aubrey H.

Robsons co-pending application Serial No. 764,170, filed September 29,1958, now Patent No. 2,979,264, it has been deleted from thisspecification to simplify explanation of this system.

It is assumed that the desired discharge air temperature has beenselected by adjusting resistor 138, and that the temperature sensed byresistor 16 is considerably below this value. Thus, current flow throughpolarized relay 84 will be of a magnitude and in a direction to closeswitch 82 to its heat contact 88. This energizes relay winding 92 whichthereby actuates switch 96 to its %a position so that power is deliveredfrom line 74 to motor 48 through line 116 and motor 48 is driven in adirection to close throttling valve 44 and thereby increase the fueldischarged from nozzle 6 into the burner. After the heated airdischarged from the heater has reached a temperature corresponding tothat selected by the resistor 138, switch 82 will break from heatingcontact 88 and move towards its neutral position since the resistance ofelement 16 and element 138 will be nearly equal and current flow through6 polarized relay winding 84 will be insufficient to maintain switch 82in contact with either the heating or cooling contact 88 or 86. Motor 48is then in a de-energized condition and throttle valve 44 will remain inthat particular position until conditions change. Since switch 95 hasremained open during this period, valve 50 has consequently remainedclosed.

If the temperature sensed by resistor 16 exceeds that temperatureselected by resistor 138, current flow through polarized relay winding84 will be in a direction to tend to close switch 82 to its coolingcontact 86. Closure of switch 82 to cooling contact 86 results inenergization of cooling relay winding 90, actuation of switch 94 to its84a position, and closure of switch 95. Motor 48 is then energized in acooling direction (opening throttle valve 44) from line 74 by way ofswitch 102 at its 1432b contact, line 122 to switch 94 at its 94acontact, line 120 to switch 104 at its 18417 contact, line 118 and limitswitch 112 in its 112!) position.

However, while throttle valve 44 is being progressively opened to causea decrease in discharge of fuel into the burner chamber, closure ofswitch 95 has completed the circuit including line 137, switch 111 andline 139 so that solenoid 51 is energized and valve 50 is opened. Thisresults in an immediate and substantial decrease of fuel discharge intothe burner chamber due to reduction in fuel back pressure at the nozzle.The temperature of the heat exchanger consequently decreases morerapidly than if the progressively opening throttle valve were aloneeffecting the reduction in fuel back pressure at the nozzle.

When the temperature sensed by resistor 16 closely approaches theselected temperature, switch 82 breaks from cooling contact 86, coolingrelay winding is deenergized, and consequently switch 94 is actuated toits 94b contact and switch opens. Thus, motor 48 and solenoid 51 arede-energized so that valve 50 is closed and fuel flow into the burnerchamber is again entirely under control of throttle valve 44. If thefuel rate into the burner is then sufiiciently close to the rate whichcontinues to produce the selected temperature, the throttle valve 44will remain in that particular position.

If by chance the throttle valve has had sufiicient time, before switch82 opens from contact 86, to open to a position resulting in aninsufiicient fuel rate, the switch 82 will close to heating contact 88and the throttle valve will be operated in a closing direction asexplained hereinbefore.

If the throttle valve 44 has not had sufiicient time to open to theproper position while valve 50 is open, the switch 82 will again closeto cooling contact 86 and valve 50 will be re-opened to by-pass fuelwhile the throttle valve opens farther.

It will be apparent then that one result of the operation of the valve50 in dumping fuel is to dampen the amplitude of outlet temperaturevariations. In effect, its operation gives the heat exchanger a headstart in cooling down when a decreased outlet temperature is required bythe control system.

If the temperature sensed by overheat sensing resistor 18 exceeds thatselected by overheat selection resistor 14%), current flow throughpolarized relay Winding 7 9 associated with overheat bridge 76 willcause switch 80 to open. Thus, relay winding 68 is de-energized,switches 71 and 72 open, and combustion is terminated.

In the operation of the heater of the present invention, it is sometimenecessary to furnish a heated air discharge having a temperature onlyslightly above the temperature of the ambient air drawn into the heater.In the attempt of the control to decrease fuel flow into the burner to asufficiently small quantity to only slightly heat the air, the motor 48will be driven to a position wherein throttling valve 44 is at itsmaximum open position. The limit switch 112 will be actuatedautomatically to its 112a position when motor 48 reaches this end travelposition. Actuation of switch 112 to its 112a position coupled withswitch 82 being in its cooling contact 86 position (thus indicating thatthe air discharged from the heater still exceeds the desired temperatureeven though the motor 48 has driven throttle valve to its maximum openposition) is used to trigger the method of operation wherein bypassvalve 38 is alternately opened and closed in accordance with thedischarge air temperature demands.

Thus, when switch 112 operates to its 112a position and switch 82remains in its cooling contact 86 position, power is delivered toenergize relay 100 by way of the following path. Line 74 delivers powerto switch 102 in its 1021) contact position, through line 122 to switch94 in its 94a position, through line 120 to switch 104 in its 104])position, through line 118 to switch 112 in its 112a position, throughline 130, switch 72 and line 132 to energize holding relay winding 100.Energization of relay winding 100 operates switches 108 and 110 to aclosed position and opens switch 111. Opening of switch 111 de-energizessolenoid 51 and thus insures that valve 51 will remain closed while fueldischarge is being controlled by bypass valve 38.

When switches 108 and 110' are actuated to their closed positions,holding relay winding 98 is also energized through switch 96 in its 9612position, line 134, switch 110, and line 136. When relay winding 98 isenergized, switches 102, 104- and 106 are all actuated to their apositions. As soon as 106 is actuated to its 106a position, the circuitto solenoid 39 is opened and valve 38, which is biased to an openposition when solenoid 39 is deenergized, moves to an open positionwhich by-passes all fuel from regulating valve 28 through primaryby-pass conduit 36 back to the tank. In other words, when bypass valve38 opens, fuel flow to the nozzle 6 through supply conducit 30 isterminated and combustion ceases.

With no fire in the burner, the outlet air temperature decreases andthis decrease is sensed by resistor 16. When the resistance of theresistors 16 and 38 tends to balance because of the decreased outlet airtemperature, current flow through the polarized relay 84 decreasessufficiently to cause switch 82 to open from cooling contact 86, relaywinding 90 is de-energized and switch 94 moves to its 94b position. Whenswitch 94 moves to its 94b position, the solenoid 39 is again energizedthrough line 128, switch 106 in its 106a position, line 126, switch 104in its 104a position, line 120, switch 94 in its 94b position, line 124,switch 102 in its 102a position, and power line 7 1-. Thus, minimumburner fire is re-established in the heater by causing a fuel dischargefrom the nozzle which is just suflicient to maintain the lowest stableflame.

If the minimum burner fire again causes the discharge air temperature toincrease above the selected value, switch 82 will again close to coolingcontact 86, and through the circuits heretobefore described, will causethe by-pass valve 38 to again open, by-pass all fuel and extinguishburner flame. This alternate open-closed operation of by-pass valve 38will continue as long as the temperature demanded by the control systemcan only be maintained by this type of operation.

It will be apparent that since switch 111 is maintained in an openposition as long as holding relay winding 100 is energized, the valve 50will remain closed irrespective of opening and closing of switch 95which is controlled by opening and closing of switch 82 to coolingcontact 86. One purpose of this arrangement for rendering valve 50ineffective to control fuel discharge during by-pass valve 38 controlconditions is to prevent unnecessary operation of this valve 50 duringthis period.

If the alternate open-closed operation does not suffice to provide thedesired air outlet temperature, this will be sensed by bridge 78 andswitch 82 will be operated by the current flow through polarized relay84 to its heating contact position 88. This will energize relay winding92 and and actuate switch 96 to its 96a position. When so actuated, themotor 4-8 is moved from its cooling end travel position by powerdelivered from line 74 through switch 96, through line 116 and limitswitch 114. As soon as motor 48 is driven from its cooling end travelposition, limit switch 112 is thereby operated to its 1121) position,the holding relay winding 100 is de-energized, and consequently holdingrelay winding 98 is also de-energized by opening switch 108. Closure ofswitch 111 in response to de-energization of holding relay winding 100provides that upon subsequent closure of switch in response to a coolingdemand, the dump valve 50 will again be operable to dump fuel. Sincesolenoid 39 is energized as soon as switch 82 opens from cooling contact86, it is insured that by-pass valve 38 is closed and a minimum burnerfire is established before switch 8 2 is moved to its heating contactposition 88. When holding relay winding 98 is de-energized, switch 106moves back to its 1061) position and solenoid 39 is again energizeddirectly from line 74 through the switch 106. This insures that when theswitch 82 moves away from its heating contact 88 (which occurs when thetemperature control bridge is again satisfied) the by-pass valve 38 willremain in a closed position as motor 48 modulates throttle valve 44 backand forth in accordance with temperature demands manifested by thetemperature control bridge 78.

By means of the described arrangement, it will be noted that switch 82operates to control fuel flow by means of dump valve 50 and by actuatingmotor 48 whenever modulation of motor 48 and valve 44 between end travelpositions will serve to satisfy the demands of the system, and alsoserves as the control for on-off fuel flow control to the burner whenthe temperature demands of the system cannot be satisfied by amodulating type of operation.

Having described my invention, I claim:

1. A fuel control system for an air heater, comprising a burner having areturn conduit; means for supplying fuel to said burner; a throttlevalve in said return conduit from said burner for varying in directrelation the fuel back pressure in said return conduit and the rate offuel discharge into said burner; a normally closed solenoid operateddump valve in parallel with said throttle valve; first electrical switchmeans actuated one way or another for energizing said throttle valve formovement in one direction or another; air temperature responsive meansfor actuating said first electrical switch means said one way or anotherfor energizing said throttle valve in a direction to increase anddecrease fuel discharged into said burner in response to a sensed outletair temperature produced by said burner respectively below and above aselected temperature; and, electrical means including circuit meanshaving second electrical switch means actuated to a closed position forenergizing said solenoid operated dump valve to an open position tosubstantially reduce said fuel back pressure in response to actuation ofsaid first switch means said other way resulting from a condition ofsaid air temperature sensitive means demanding a decreased rate of fueldischarge into said burner.

2. A fuel control system for an air heater, comprising: a burner havinga nozzle; a main conduit for carrying fuel to said burner nozzle; pumpmeans for supplying fuel under a predetermined pressure to said mainconduit; return conduit means connected to said burner nozzle forreturning excess fuel; a throttle valve in said return conduit means forcontrolling the rate of fuel discharge into said burner; air temperatureresponsive means for controlling said throttling valve, said means beingoperable to open and close progressively said throttling valve inresponse to a sensed outlet air temperature produced by said burnerrespectively above and below a selected temperature; a dump valve inparallel to said throttling valve in said return conduit means; andmeans for opening said dump valve in response to a sensed airtemperature above said selected temperature to substantially reduce fuelflow into said burner while said sensed temperature produced by saidburner exceeds said selected temperature.

3. The system of claim 2 including: a bypass conduit connecting saidmain conduit to said return conduit means; a valve in said bypassconduit; means for opening said bypass conduit valve in response to amaximum open position of said throttling valve with a coincidentalcondition of said air temperature responsive means corresponding to saidsensed temperature being produced by said burner above said selectedtemperature; and means for maintaining said dump valve in a closedposition While said throttling valve is in said maximum open position.

4. A fuel control system for an air heater, comprising: a burner havinga bypass type nozzle; conduit means for supplying fuel to said burnernozzle; fuel return line means connected to said burner nozzle; amodulating valve in said return line means operable in a first directionto increase progressively the rate of fuel discharge into said burner,and in the opposite direction to decrease progressively the rate of fueldischarge into said burner; air temperature responsive means foroperating said modulating valve in said first direction in response to asensed air temperature produced by said burner below a desired airtemperature, and in said opposite direction in response to a sensed airtemperature produced by said burner above said desired air temperature;a solenoid valve in said re turn line means in parallel with saidmodulating valve; a circuit for energizing said solenoid valve; andswitch means operative in response to a condition of said airtemperature responsive means causing operation of said modulating valvein said opposite direction, for completing said circuit to open saidsolenoid valve for reducing the rate of fuel discharge into said burnerindependently of the position of said modulating valve.

5. The fuel system specified in claim 4 wherein: said solenoid valveincludes means biasing it to a closed position when said solenoid valveis not energized.

6. The fuel system specified in claim 5 including: second switch meansoperative, in response to a maximum open position of said modulatingvalve, to open said solenoid valve circuit.

7. A fuel control system for an air heater, comprising: a burner; supplyconduit means for supplying fuel to said burner; return conduit meansconnected to said burner for fuel bypassing said burner, said returnconduit means including parallel fluid flow paths; a throttle valve inthe first of said parallel flow paths; air temperature responsive meansoperative to modulate said throttle valve towards an open position inresponse to an air temperature produced by said burner above a desiredair temperature, and towards a closed position in response to an airtemperature produced by said burner below said desired air temperature;a two-position valve in the second of said parallel flow paths; andmeans for operating said two-position valve to an open position whensaid throttle valve starts to modulate in an opening direction, andmaintaining said open position while said modulation continues.

References Cited in the file of this patent UNITED STATES PATENTS1,973,765 Hunt Sept. 18, 1934 2,209,926 McGrath July 30, 1940 2,249,844Martin July 22, 1941 2,758,591 Hubbard Aug. 14, 1956 2,816,605 SevilleDec. 17, 1957

